This application claims the priority of Japanese Patent Application No. 9-281397 filed on Sep. 29, 1997 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an arrangement of a signal transmission circuit for a solid-state image pickup device, and more specifically a configuration of a transmission circuit which transmits video signals obtained with a solid-state image pickup device to a signal processor unit or the like several meters apart therefrom.
2. Description of the Prior Art
An electronic endoscope system comprises a CCD (charge coupled device) which is a solid-state image pickup device built in a tip of an electronic endoscope used as a scope, picks up an image in an observed body with the CCD and displays the image on a TV monitor or the like. An electronic endoscope system of this kind has a configuration wherein the electronic endoscope is connected to the processor unit through a cable so that the CCD is connected to a signal processing circuit in the processor unit by way of a transmission line 2 to 3 m long. Therefore, a signal transmission circuit is used to transmit video signal outputs of the CCD favorably with no loss.
Object of the Invention
To transmit signals favorably while taking degradation of an S/N ratio into consideration, the applicant proposed a signal transmission circuit of cascade connection, for example, as that shown in FIG. 3 in a U.S. patent filed on Jun. 26, 1998. In FIG. 3, an electronic endoscope 1A is connected to a processor unit 3A by way of a transmission line (cable) 2, a CCD 4 is disposed in a tip of the electronic endoscope 1A, a CCD driver 5 and a CCD bias circuit 6 are connected to the CCD 4 so that the CCD bias circuit 6 supplies a bias voltage to the CCD 4 and the CCD driver 5 drivers the CCD 4 to reads out electric charges accumulated in the CCD as video signals.
An output transistor 8 is disposed to output the video signals obtained with the CCD 4 to the transmission line 2. The transistor 8 has a base which is connected to an output side of the CCD 4, an emitter which is grounded by way of a bias resistor R1 and a collector which is connected directly to the transmission line 2.
On the other hand, disposed on a side of the processor unit 3A is an input transistor 9 which has an emitter connected to the transmission line 2, a base grounded by way of a DC power source 10 and a collector connected to a voltage source +Vcc by way of a resistor R2. This transistor 9 functions as an base follower. Further, a buffer transistor 11 is connected to a connection point between the collector of the transistor 9 and the resistor R2, an emitter of this transistor 11 is grounded by way of a resistor R3, and a signal processing circuit 12 is disposed at a connection point between the emitter and the resistor R3.
Calculating from an input signal of the output transistor 8 and an output signal of the input transistor 9 obtained in the configuration described above, a gain G of the output voltage of the transistor 9 relative to the input voltage of the transistor 8 is determined as G=R2/R1.
The output transistor 8, the transmission line 2, the input transistor 9 and the DC power source 10 compose the so-called cascade connection which allows the video signals to be transmitted as current variations. Speaking more concretely, the transmission line 2 which is connected to the emitter of the transistor 9 has extremely low AC impedance, thereby allowing no AC voltage to be generated therein and transmitting signals as current variations taking a bias current as standard. This configuration is capable of preventing an S/N ratio and a frequency characteristic from being degraded due to variations of a signal voltage transmitted through the transmission line 2.
In the circuit shown in FIG. 3, however, it is necessary to incessantly supply the bias current for an operation of class A, whereby more or less heat is produced from the transistor 8 and the resistor R1 in an output section of the CCD 4. It is preferable to reduce this heat as far as possible since it accelerates heating of the tip of the endoscope and constitutes a cause for noise.
The present invention has been achieved in view of the problem described above and has an object to provide a signal transmission circuit for a solid-state image pickup device which is of a type different from the signal transmission circuit proposed by the above-mentioned U.S. patent and is capable of favorably transmitting signals while suppressing heat production to a minimum level around the image pickup device.
To accomplish the object described above, an invention as claimed in claim 1 provides a signal transmission circuit for a solid-state image pickup device which is used for transmitting output video signals from a solid-state image pickup device to a signal processor by way of a transmission line, characterized in that a complementary circuit which performs an operation of class B by alternately operating two transistors is disposed as a signal output circuit on a side of the solid-state image pickup device.
An invention as claimed in claim 2 provides a signal transmission circuit for a solid-state image pickup device, characterized in that the solid-state image pickup device is connected to a ceramic circuit board, and a signal output circuit which comprises the complementary circuit is configured as an integrated circuit and mounted directly on the ceramic circuit board.
In the configuration described above wherein the two transistors of the complementary circuit operate alternately, video signals are output from the solid-state image pickup device as positive and negative signals taking 0 V (a bias voltage may be applied), for example, as standard. Such an operation of class B does not supply a bias current unlike the operation of class A, thereby preventing heat production from the transistors and resistors mentioned above due to application of a bias current.
When the complementary circuit is mounted directly on the ceramic circuit board, it is also possible, owing to a heat radiating effect of the ceramic circuit board, to efficiently radiate heat produced due to signal transmission other than the heat produced due to application of the bias current, thereby preventing surroundings of the solid-state image pickup device and the tip from being heated.